NULLIF function is not NON_NULLABLE nor NULLABLE

[jeeminso]

Summary of the changes / Why this improves CrateDB

Fixes #15325

By definition

   - Nullable means the function returns a null value for a null argument.

   - Non-nullable means the function returns never a null values for null
     arguments.

   - Conditional means it follows it's own logic. By default each scalar
    function is conditional.

NULLIF is conditional.

---

Without the fix:

cr> SELECT * FROM t0 WHERE (NOT NULLIF((t0.c1), NULL));
+------+
| c1   |
+------+
| NULL |
+------+
SELECT 1 row in set (0.106 sec)

With the fix:

cr> SELECT * FROM t0 WHERE (NOT NULLIF((t0.c1), NULL));
+----+
| c1 |
+----+
+----+
SELECT 0 rows in set (3.826 sec)

Checklist

• Added an entry in the latest docs/appendices/release-notes/<x.y.0>.rst for user facing changes
• Updated documentation & sql_features table for user facing changes
• Touched code is covered by tests
• CLA is signed
• This does not contain breaking changes, or if it does:
  • It is released within a major release
  • It is recorded in the latest docs/appendices/release-notes/<x.y.0>.rst
  • It was marked as deprecated in an earlier release if possible
  • You've thought about the consequences and other components are adapted
    (E.g. AdminUI)

[matriv]

thx for the quick fix @jeeminso !